This month in Poland, President Obama offered $1 billion in military assistance to address our NATO allies' anxiety about a resurgent Kremlin. Labeled the European Reassurance Initiative, this boosts NATO's budget by a mere 0.1%. It was nonetheless received with enthusiasm by European partners happy to have a Band-Aid to cover up an unpleasant truth: For decades, our NATO allies have so underinvested in creating their own security forces that when Vladimir Putin moved 40,000 Russian troops to Ukraine's border, they had no capacity to respond.

The crisis in Ukraine reminds us of dangers that are too easy to forget. Obama missed an opportunity to borrow a line from President Kennedy's 1961 inaugural address. He should have challenged the leaders of Britain, France, Germany and even Poland to ask not what Americans can do for European security, but to ask what they can do for themselves.

This is not an abstract question. Two decades after the end of the Cold War, the twin trends of overdependence on U.S. power and underinvestment in military might have left European defenses at risk of becoming dangerously irrelevant.

The European Union's population is triple Russia's and its economy is eight times larger, yet it spends 60% less on defense than Russia does relative to GDP. Such miserly investment helps explain Europe's impotence in response to Putin. For Washington, this state of affairs is untenable. For Europe, it should be unacceptable.

This month's commemoration of D-day was a fitting reminder of American sacrifices for European security. On June 6, 1944, 73,000 Americans landed on the beaches of Normandy to take the ground war to the European continent. Before Germany surrendered 11 months later, more than 180,000 Americans had given their final measure in Europe. Had Americans not come to their rescue, Europeans could still be living under the jackboot of one of the most brutal regimes in history.

The twin trends of overdependence on U.S. power and underinvestment in military might have left European defenses at risk of becoming dangerously irrelevant.

World War II left Europe in ruins. In what British Prime Minister Clement Attlee called an "act of unparalleled generosity and statesmanship," the U.S. contributed more than 5% of its GDP (the equivalent of $840 billion today) to reconstruct Europe.

Later, when Soviet troops occupied the nations of Eastern Europe and the Soviet sector of Berlin and threatened to undermine Western European states, Americans responded. NATO became the greatest military alliance of all time, complete with an American nuclear umbrella to deter Soviet nuclear threats, even though this meant risking Boston for Berlin.

When the Berlin Wall came tumbling down in 1989, Americans took the lead in helping Germany recover East Germany and emerge as a unified state in the EU and NATO. When the Soviet Union collapsed in December 1991, Americans again took the lead, working with the new Russia to eliminate nuclear weapons left elsewhere in the former Soviet Union and secured the weapons that were returned to Russia.

Moscow was persuaded to recognize the independence and sovereignty of formerly occupied Eastern and Central Europe and the emerging states of the former Soviet Union. The U.S.-led NATO admitted 12 states from Eastern and Central Europe to its ranks, including Poland and the Baltic states, sheltering them under the security umbrella. To this day, Americans risk their lives to protect the security of citizens in Europe.

This defense is not cheap, and it is not balanced. Americans spend about $2,300 per head on defense, including the defense of Europe. Europeans spend $550 per head on their own defense.

On paper, this adds up to $275 billion in total European defense spending and 1.5 million active duty European troops. This is almost twice the number of active-duty troops as Russia has and three times its budget. But the gap in what defense experts call "tooth to tail" — the amount devoted to overhead and support for each combat soldier — is evident in the test that was posed by Russian troops on the border of Ukraine. What did European NATO members do? What they have always done: Call USA-911. Truth be told, European military forces today sometimes seem like expensive dentures.

Asking America to provide a security blanket has an understandable appeal. But for war-weary Americans determined to reduce unsustainable deficits by cutting federal expenditures, including defense, the current arrangement appears increasingly anachronistic.

As the Ukraine crisis reminded Europe's leaders about threats on their continent, it is past time for Europeans to ask less what America can do for them, and more what they can do for European security.

Graham Allison is director of the Belfer Center for Science and International Affairs at Harvard University's John F. Kennedy School of Government.